In order to upgrade or improve the appearance of wood panel surfaces, particularly with respect to finish, adhesion and durability, such surfaces are often covered with an overlay of resin impregnated paper. The resultant panel, however, usually has a smooth, somewhat uninteresting appearance. It has therefore also been common to incorporate a texture or relief to the overlaid panel surface, thereby lending a three-dimensional effect.
One method used to provide such a three-dimensional effect is the machining of grooves into the surface of the panel. However, in addition to the high costs incurred in the machining operation, the grooves formed in the panel by such machining contain areas of exposed wood which present problems of paint durability. In addition, an extensive or overall textured pattern is not provided by such machining.
Because of the shortcomings of machining, a technique whereby wood grain textures are embossed into the overlaid wood surfaces has been preferred. In this procedure, a caul having the desired texture or design and the panel sought to be embossed are placed between the platens of a hot press. The caul and panel are compressed under heat and pressure, so that the panel takes on and retains the relief impression of the caul.
Heretofore cauls made of metal, generally steel, aluminum or nickel, have been employed in a platen press for embossing. These cauls are generally textured or provided with the desired configuration by one of several manufacturing techniques. The most common technique involves the construction of a hardened metallic master tool, from which a number of individual cauls can be fashioned from relatively softer metals by the combined application of pressure and an acid etching agent. Another method involves the application of an etching agent to specified areas of a metallic surface, resulting in controlled removal of metal from the surface to produce a desired texture. Still another technique involves caul formation by electrochemical deposition of nickel onto a textured surface which has been coated with an electrically conductive primer. Lastly, there is a technique whereby a textured caul plate is formed by pouring molten aluminum into a suitable mold (sand casting).
It can readily be appreciated that in addition to the cost of the metal, the cost of producing a desired texture by any of the aforementioned techniques is relatively expensive. This cost of manufacturing tends to limit the number of different caul designs that a manufacturer can afford to use. This is obviously a significant limitation when one considers the number of potential designs that are available. It can also be appreciated that some of the methods by which metal cauls are produced--i.e., etching and casting--often yield a less than genuine reproduction of a wood grain or other texture. Another shortcoming attendant with the commercial metal cauls currently available is that they are somewhat heavy and can be difficult to handle, especially during installation and removal from a hot press.
An alternative caul construction method is taught, for example, in U.S. Pat. No. 3,454,457 to Hale et al. This method involves a fabricated caul plate made up of resin filled sheets with a synthetic resin film release sheet thereon. In Hale et al the resin filled sheets are impregnated paper and as such are subject to a problem of thermal degradation during use. Thermal degradation of paper (wood) is reported and explained by Dr. E. L. Schaffer of the Forest Products Laboratory, Forest Service, U.S. Department of Agriculture in a paper entitled, "Smoldering Initiation in Cellulosics under Prolonged Low-Level Heating." Hale et al resin impregnated paper also has shortcomings when used in a caul since it has lower thermal conductivity requiring longer curing times for wood panel adhesives and therefore lower rates of panel production than comparable thicknesses of resin impregnated fiberglass or graphite. Also, a composite steel and impregnated paper caul would combine two materials with significantly different thermal contraction and expansion resulting in high thermal stresses and possible delamination when cycled between service and storage temperatures. It is desirable, however, to have composite structures for flexural stiffness and thermal conductivity when the caul is to be used as a "hanging" caul. "Traveling" cauls, on the other hand, move into and out of the press with the panels and do not require the resistance to flexure that hanging cauls do. Accordingly, they need not include a metal core or backing sheet, but if made of resin impregnated paper, they still have the disadvantages of requiring comparatively longer press cycle times due to poorer thermal conductivity, and of being subject to thermal degradation during use.